A conventional apparatus for the continuous casting of metals includes a cooled mold having a casting passage which extends in a generally vertical direction and which has open upper and lower ends. Molten metal is continuously introduced into the upper end of the casting passage from a tundish and forms a bath in the mold. The molten metal adjacent the walls of the casting passage solidifies to form a shell which surrounds the bath. A strand consisting of the shell and a molten core is continuously drawn out of the lower end of the casting passage via a strand withdrawal mechanism. The strand is sprayed with a cooling fluid, typically water, outside of the casting passage so as to progressively solidify its molten core.
During the withdrawal of the strand from the mold, the latter is oscillated parallel to the casting direction. This prevents the strand from sticking to the walls of the casting passage.
It is important to maintain the bath level in the mold within prescribed limits. If the bath level becomes too high, there is the danger of overflow. If the bath level becomes too low, the thickness of the shell upon leaving the mold will be less than that required to withstand the pressure generated by the molten metal within the shell.
The bath level may be controlled in several ways. It is possible to regulate the rate at which molten metal flows into the mold. This is normally achieved by providing the tundish with a stopper rod and manipulating the latter to increase or decrease the rate at which molten metal flows out of the tundish. Another possibility is to regulate the rate at which the strand is drawn out of the mold. This may be accomplished by controlling the speed of the strand withdrawal mechanism. It is further possible to regulate the inflow of molten metal into the mold as well as the withdrawal of the strand from the mold.
Since it is not always convenient to observe the bath level visually, detectors for monitoring the bath level without visual aid have been developed. One such detector includes a radioactive source and a receiver which are located on opposite sides of the mold. The radiation from the source travels to the receiver and the change in intensity of the radiation as it traverses the mold provides an indication of the bath level. Another such detector employs an induction unit which generates a primary electromagnetic field. The latter, in turn, induces currents in the mold walls which result in secondary electromagnetic fields. The magnitudes and directions of the secondary electromagnetic fields are indicative of the bath level.
A detector such as above may be connected with a control unit which interprets the signals received from the detector and controls the tundish stopper rod and/or the strand withdrawal mechanism accordingly.
The detector, which monitors the bath level continuously, is sometimes mounted on the mold since this places it in close proximity to the bath and thereby results in more reliable readings than would otherwise be the case. However, the detector then oscillates relative to the bath so that any two consecutive readings are taken at two different positions of the detector. Even if the bath level remains perfectly stationary, the detector then senses a change in bath level due to its own movement and initiates an adjustment via the control unit. This oscillation effect, which makes it difficult for the detector to distinguish between its own movement and a shift in the bath level, leads to unstable operation.
In order to overcome the oscillation effect, it has been suggested to incorporate a timer in the control unit. The timer causes the control unit to accept signals from the detector at predetermined time intervals only. Thus, although the detector is continuously monitoring the bath, the control unit receives signals only when the timer is activated. The timer may, for example, be set to operate once during each oscillation cycle. The timer is synchronized with the oscillator so that is is activated at the same instant during each oscillation cycle. For a constant oscillation rate, the detector will then always be at the same position when the signals therefrom are accepted by the control unit. Accordingly, the oscillation effect is eliminated.
The timer system works satisfactorily so long as the oscillation rate remains constant. However, since the timer cannot be reset during a casting operation, the same problem as previously arises when the oscillation rate must be changed during a casting operation due to changes in the casting conditions. The timer is then no longer synchronized with the oscillator so that the position of the detector is different whenever the timer is activated.
Another known system avoids this shortcoming of a timer. Here, a radioactive detector mounted on a generally vertical mold has a receiver which forms part of an electrical circuit. The radiation reaching the receiver generates a voltage which is proportional to the intensity of the radiation and which is continuously changing due to oscillation of the mold. The latter is oscillated by an arm having a roller which rides on top of a horizontal, eccentric shaft mounted for rotation. As the shaft rotates, the arm moves up and down thereby imparting an oscillatory movement to the mold. The arm is connected to the slide of a potentiometer which is in circuit with the receiver. By virtue of this connection, the arm produces a voltage which is dependent upon the position of the arm. The voltage due to the arm is in opposition to that generated by the radiation. These voltages are fed into a compensating unit which calculates the mean value thereof. This value, which is indicative of the bath level, is read on a dial or other instrument and may be used to control the tundish stopper rod and/or the strand withdrawal mechanism.
Instead of using a potentiometer as the voltage-generating unit for the arm, it is possible to use a magnetic arrangement consisting of an iron plate which moves between two poles in synchronism with the mold.
Although the latter system avoids the disadvantage of a timer outlined earlier, other problems arise with this system. To begin with, the voltage-generating unit for the arm must be relatively accurately zeroed if reliable compensation for the mold oscillation is to be achieved. This condition is difficult to maintain in a steel mill environment. Furthermore, the arm and its voltage-generating unit must be mechanically linked with one another. If looseness develops in the linkage, as is apt to occur, the compensation will no longer be reliable. Additionally, monitoring of the bath level and compensation for mold oscillation are performed continuously during a casting operation. Since the resulting voltages are fed into the compensating unit on a continuous basis, the compensating unit performs calculations continuously during each casting operation which affects its life adversely.